Tornadoes: Questions and Answers

At The Old Farmer's Almanac, readers often ask us about tornadoes. Here are the most common questionsand answers about nature's most violent storms!

Q. What exactly is a tornado?

A. A tornado is a violently rotating column of air between a cloud and Earth, touching both. When conditions are right, a thunderstorm can spin out one or more tornadoes. There must be moisture in the low to middle levels of the atmosphere, air that is rising from the ground and strong enough to keep rising, and a "lifting force" that causes the air to begin rising. (This happens when air near the ground is warmed.)

As air rises, it cools, and the moisture in it begins to condense, forming a cloud. If the lifting force is strong enough and the air has enough moisture, this cloud can tower more than 50,000 feet. The updraft can carry winds upward of 100 mph. Tornadoes form in this updraft. Falling rain or hail pulls air down to form downdrafts. Since the tornado is in rising air, the wind around it is flowing into the tornado. Damaging winds can hit hundreds of yards from the tornado's vortex.

Q. If there is one tornado, are there likely to be more?

A. Tornadoes can come one at a time or in clusters, and they can vary greatly in length, width, direction of travel, and speed. They can leave a path 50 yards to more than a mile wide. They may touch down for seconds or remain in contact with the ground for much longer periods, although the average is about 15 minutes. Scientists are studying tornadoes so that they can more accurately predict what conditions create them and thus improve warning techniques.

Q. Is a twister the same as a tornado?

A. The term twister has been used for some smaller tornadoes, and some people use the term interchangeably with tornado. Meteorologists frown on such usage.

Q. Where do the most tornadoes occur?

A. The United States has the highest incidence of tornadoes worldwide, with about a thousand occurring every year. The country's unique geography brings together polar air from Canada, tropical air from the Gulf of Mexico, and dry air from the Southwest to clash in the middle of the country, producing thunderstorms and the tornadoes they spawn.

Tornadoes have hit all 50 U.S. states, but some states have tornadoes more often than others. Oklahoma, Texas, and Florida are the leading candidates. Frequent thunderstorms in western Florida contribute significantly to the number of tornadoes there, but the tornadoes tend to be relatively weak. Fewer thunderstorms occur in the central Great Plains, but tornadoes spawn there frequently because of the low-level wind shears in that part of the country.

Q. Is there a tornado season?

A. The combination of conditions that cause tornadoes is common across the southern United States in the early spring. As the season goes on, tornadoes are likely to occur farther and farther north in the plains states and the Midwest. April and May tornadoes are common in the South as well as in those more northern areas. Often a large storm system can create tornado conditions for several days in a row.

Q. How are tornadoes measured?

A. The Enhanced Fujita Scale (EF Scale), developed by Dr. T. Theodore Fujita at the University of Chicago, assesses tornado damage. The scale is a set of wind estimates (not measurements) based on damage. Specifically, it uses three-second gusts estimated at the point of damage based on a judgment of 8 Degrees of Damage (DOD) to 28 Damage Indicators. The DOD reflects the speed of the wind gusts (mph). The Damage Indicators reflect the type of structure/typical construction of what has been damaged (e.g., one or two family residence with brick veneer siding).

Q. They say the sky turns green before a tornado. Is this true?

A. Clouds of severe storms often take on a greenish hue. No one knows for certain why this happens, and it could be due to a combination of phenomena. One theory suggests that, as storms usually develop in the afternoon, the longer wavelengths (red and yellow) of afternoon sunlight (already deficient in blue), turn the bluish water-heavy clouds green.

Q. Why do tornadoes always hit mobile home parks?

A. Tornadoes do not hit mobile homes more frequently than they hit other structures. It's just that mobile homes are more vulnerable to tornado damage. Mobile homes can easily be overturned by strong wind gusts. In addition, their thin walls make them susceptible to windblown debris. Mobile homes do not have cellars in which to seek shelter. Residents of mobile home parks in tornado-prone areas should never attempt to remain in their homes when a tornado approaches. Many parks have designated community shelters, and those that don't, should.

Q. If you are not near a storm shelter or cellar when a tornado approaches, what's the best thing to do?

A. The normal lead time for tornado warnings is no more than 20 minutes, and typically less than that, so sometimes there aren't a lot of options when sturdy shelter is not nearby. Tornadoes harm people primarily through flying debris. Being outdoors when a tornado strikes poses a threat if there are many things nearby that could go flying through the air, so it's best to be in an open field if possible. Another good, but not great, place is in a ditch. It's a really bad idea to stay in your car or to get into a car to get away from a tornado. Cars are death traps in these storms, as they can be flung about by high winds or crushed by debris.

Indoors, a small room in the interior of your home, such as a closet or bathroom, offers the best protection next to a cellar. The key is to stay away from windows.

Tornado Tales from The Old Farmer's Almanac Book of Weather and Natural Disasters

In the Great Tri-State Tornado, which hit Missouri, Illinois, and Indiana in 1925, Joe Boston, a policeman in Murphysboro, Illinois, lost a bond for a deed that had been locked inside a safe in his home. The bond was sucked out of the safe and transported to the town of Lawrenceville, 125 miles northeast of Murphysboro. There it was found and returned to Boston by mail. About the same time, 130 miles away from Murphysboro in Robinson, one lucky farmer watched a $10 and a $20 bill drop right out of the sky, along with other articles that the tornado had swiped from Murphysboro.

Q. Was Elvis ever in a tornado?

A. Yes! When Elvis was 2 years old, reportedly, a tornado swept through his hometown of Tupelo, Mississippi, on April 5, 1936. The storm was devastating, destroying homes across the street from the Presleys', but they escaped unscathed. They say a Tupelo kitchen was blown, intact, to the town of Mooresville, 7 miles away, and that the tornado sucked the feathers off Tupelo chickens. Tupelo lost 235 people in that tornado.

Links to More Information

For a comprehensive discussion of tornadoes, visit the Web site of the National Oceanic and Atmospheric Administration (NOAA), which gives daily tornado and severe weather reports and compiles tornado statistics from recent years. Included there are safety and shelter tips and a discussion about tornado warning systems.

By michael mcgovern

I have been dwelling on the fluid dynamics of tornados and how they form.
I am confused on one point. Is hail formed on the upper surface of the cone as the cell spreads and falls in
or is it formed in the cell itself as a byproduct of the cell vortex
If it is the funnel surface that would imply an incredible level of surface tension. I find the relationship between surface tension and weather formation very fascinating. Has research in this area been done and
if so is it available in laymen's terms.

By Clarissa Medina

While many like to joke that "mobile homes attract tornadoes," there is no meteorological or scientific basis to thinking that that theory. In fact, the explanation for the reports of damage to manufactured homes from tornadoes is quite simple: manufactured housing is largely found in rural and suburban areas where tornadoes are most likely to occur.

As to hurricanes, valuable lessons were learned from the devastation of Hurricane Andrew in 1992, which destroyed or damaged thousands of site-built and manufactured homes. Now, in areas prone to hurricane-force winds, the standards for manufactured homes are equivalent to or more stringent than the current regional and national building codes for site-built homes in these high wind zones.

By don parham

As one who was born and raised in Oklahoma, I've lived through many tornado warnings. One thing I observed about tornadoes is that most of the time they came around or just after sunset. I learned that this has to do with the "dry line" that moves east during the day. Can you explain this phenomenon?